1. Technical Field of the Invention
The present invention relates generally to tire inflation pressure sensing apparatuses or sensors. More particularly, the invention relates to a direct-type tire inflation pressure sensing apparatus for use in an automotive vehicle, which has a function of identifying a spare wheel of the vehicle.
2. Description of the Related Art
Conventional direct-type tire inflation pressure sensing apparatuses generally include a plurality of transmitters and a receiver.
Each of the transmitters is directly installed to one of a plurality of running wheels of a vehicle and includes a pressure sensor working to sense the inflation pressure of a tire mounted on the running wheel. Each of the transmitters is configured to send out a pressure signal representative of the inflation pressure of the tire sensed by the pressure sensor.
The receiver is installed to the body of the vehicle and includes a plurality of antennas each of which corresponds to one of the transmitters. The receiver is configured to receive the pressure signals sent out from the transmitters through the respective antennas and determine the inflation pressures of the tires based on the received pressure signals.
With the above arrangement, when the receiver receives all the pressure signals sent out from the transmitters at the same time, it cannot identify from which one of the transmitters each of the received pressure signals is sent out.
In order for the receiver to identify the pressure signals, each of the transmitters may be configured to send out the pressure signal upon being triggered by a trigger signal. At the same time, the receiver may be configured to sequentially send out the trigger signals, one to each of the transmitters, through the respective antennas, so that it can receive the pressure signals sent out from the transmitters at different times. As a result, the receiver can identify each of the pressure signals sent out from the transmitters and accurately determine the inflation pressure of each of the tires based on the corresponding pressure signal.
However, in the above case, the receiver may receive more than one pressure signal at the same time when a spare wheel of the vehicle is located close to one of the running wheels.
Specifically, the spare wheel is generally accommodated in a rear trunk of the vehicle, so that it can be positioned close to one of the antennas that corresponds to one of the transmitters on the rear running wheels of the vehicle. Thus, in some cases, when the receiver sends out the trigger signal through the antenna, both the transmitters on the rear running wheel and spare wheel of the vehicle are triggered to send out the respective pressure signals. As a result, the receiver receives two pressure signals at almost the same time, so that it cannot identify whether each of the two pressure signals is sent out from the transmitter on the rear running wheel or that on the spare wheel.
To solve such a problem, an approach is proposed in Japanese Patent First Publication No. 2003-306017, an English equivalent of which is International Publication WO03086787. According to the approach, both the transmitters on the rear running wheel and spare wheel of the vehicle continuously send out the respective pressure signals during at least one revolution of the running wheels. The receiver then monitors both the continuously sent out pressure signals and identifies each of those based on the monitoring results. Specifically, if one of those pressures signals varies with rotation of the running wheels, it is sent out from the transmitter on the rear running wheel; Otherwise, it is from the transmitter on the spare wheel.
However, continuously sending out the pressure signals increases electrical power consumption of the transmitters. For example, when the vehicle runs at a general speed of 60 km/h, it takes about 0.1 second for the running wheels of the vehicle to complete one revolution. Keeping transmissions of the pressure signals for such a long time only for the purpose of spare wheel identification is very disadvantageous to the lives of the batteries provided in the transmitters.
Further, for the purpose of spare wheel identification, one may consider applying other special signals sent out from the transmitters to the receiver, instead of the pressure signals. However, this will require additional devices and make the structure of the tire inflation pressure sensing apparatus unnecessarily sophisticated, thus increasing manufacturing cost of the sensing apparatus.